Many systems exist by which a graphic artist can produce drawings on a computer screen with sophisticated effects such as shading, stippling, and painting-like color variations. It has not been previously possible, however, to produce on a computer screen the visual effects which an artist would expect to see as he works in watercolor media. For example, in watercolor painting, the appearance of a brush stroke is dependent upon such factors as the humidity and color of the paper, the dilution and quantity of the paint, the material and texture of the brush, and the smearing, bleeding and mixing of colors when applied to wet paper.